The U.S. Supreme Court on Monday refused to take up an Oklahoma Supreme Court ruling that said a proposal to grant "personhood" to human embryos would be an improper ban on abortion.

The proposed constitutional amendment, which was never considered by voters, would have given human embryos the rights and privileges of citizens in Oklahoma and was called "clearly unconstitutional" by the state Supreme Court in an April ruling.

The measure was challenged by the American Civil Liberties Union and the New York-based Center for Reproductive Rights on behalf of several Oklahoma doctors and residents before it could be placed on the ballot.

"Today's rejection by the highest court in the nation is yet another resounding message to the opponents of reproductive freedom that such extremist assaults on our fundamental rights will not stand," Nancy Northrup, president and CEO of the Center for Reproductive Rights said Monday.